1. Field of the Invention
This invention relates to a robot apparatus, and a load absorbing apparatus and method for absorbing load applied to a motor, and specifically, to a robot apparatus, and a load absorbing apparatus and method for motors which are used as actuators for joint motion of a multi-joint type robot.
For more detail, this invention relates to a robot apparatus, and a load absorbing apparatus and method for appropriately detecting and controlling overload which is applied to a motor and may break the motor or deform the body of the robot apparatus, and more specifically, to a robot apparatus, and a load absorbing apparatus and method, which are applied to a multi-joint type robot comprising a plurality of actuator motors in order to prevent breakage of members and the body due to overload applied to the motors having a single or multiple axes.
2. Description of the Related Art
“Robot” is a mechanical apparatus which performs like human beings, with electrical or magnetic functions. This term “robot” is said to be originated from “ROBOTA (slave machine)” in Slavic. In Japan, robots were started to be spread in the late 1960s, and many of them were industrial robots such as manipulators and carrier robots for automated and unmanned factories. However, recent development relating to two-legged walking robot have been highly expected to be put to practical use. Specifically, the two-legged walking robot which is modeled after human motion is called humanoid robot.
As compared with crawler, four-leg, or six-leg type robots, two-leg type robot is unstable when moving, and its posture and walking are hard to be controlled. However, the two-leg type robot is capable of waking on uneven floors and going up and down steps and ladders, which is an advantage.
This kind of two-legged walking robot generally has a degree of freedom in many joints and these joints are moved by actuator motors. That is, a motor's output shaft is connected to one end of a link composing a part such as an arm or a leg, via a reduction gear, and the other end of the link is connected to a motor for next-joint motion. In order to realize desired performance and postures, servo control is performed based on the rotational position and rotational amount of each motor.
In general, a servo motor is used for realizing a degree of freedom in a joint of a robot, because it is easy to use, it is compact and has high torque, and it is superior in response. Specifically, an AC servo motor is brushless and maintenance free, so that it can be applied to an automated machine which is desired to work in unmanned working space, for example, to a joint actuator of a legged robot which walks freely. With a rotor comprising a permanent magnet and a stator consisting of multi-phase (for example, three-phase) coil windings, the rotor of the AC servo motor produces rotary torque with a sine wave flux distribution and a sine wave current.
Advanced two-legged walking robot autonomously walks and moves. In addition, this robot is capable of standing up from a lying position and holding and carrying objects with its arms. On the other hand, overload may be applied to its joint actuators when it falls down, bumps against something, and gets something into its body.
Such overload may cause fatal damage, for example, breakage or plastic deformation of its body. Therefore, what is crucial is that each motor constituting a joint actuator is provided with a mechanism for absorbing load.
FIG. 1 shows a simple robot model. That is, the robot drives a motor 120 under the control of a higher-ranked controller not shown, and gives output torque to a link 122 via a gear 121, so as to move a movable part.
In this figure, a torque limiter is provided between the gear 121 and the link 122, in order to absorb shocks to be given from the outside to the link 122, thereby being capable of previously preventing breakage of the motor 120 and so on, caused by the shocks, such as deformation of the output shaft of the motor 120.
Various kinds of torque limiters (or servo savers) have been proposed (for example, refer to Japanese Patent Laid Open No. 60-192893). FIG. 2 shows one example of the torque limiters.
In a torque limiter 130 of this figure, first and second semicircular friction plates 132A and 132B are arranged inside a ring 131 fixed to a link 135. These first and second friction plates 132A and 132B are fixed to the output shaft 134 of a motor via elastic material 133 such as rubber or compression coil springs and are pressed against the ring 131 by a fixed pressure caused by the elastic material 133.
In this torque limiter 130, the ring 131 can be generally rotated together with the output shaft 134 of the motor by frictional force generated between the first and second friction plates 132A and 132B and the ring 131. However, when load greater than static friction force between the first and second friction plates 132A and 132B and the ring 131 is applied to the ring 131 due to a shock applied to the rink 135, the ring 131 and the first and second friction plates 132A and 132B slip on each other, so as not to cause load greater than kinetic friction force between the ring 131 and the first and second friction plates 132A and 132B, in the output shaft 134 of the motor.
This conventional torque limiter 130, however, has a problem in that static friction coefficients between the ring 131 and the first and second friction plates 132A and 132B vary easily and widely, so that it is difficult to determine an allowable margin at the time of designing a robot.
Further, in the conventional torque limiter 130, the static friction coefficients between the ring 131 and the first and second friction plates 132A and 132B vary easily depending on temperature, which is also a problem.
Furthermore, since the conventional torque limiter 130 is constructed mechanically as described above, it is hard to construct a smaller and lighter torque limiter and to therefore realize a smaller and lighter robot which contains motors.